It is a problem in the field of data storage systems to minimize the data retrieval time when reading a data record from a data storage system. In disk drive memory systems for example, a processor is typically connected by a channel interface unit to a plurality of data channels. Each data channel is connected to one or more Direct Access Storage Device (DASD) units which function to store large quantities of data. Each DASD unit includes a control unit for interfacing with the data channel that typically carries eight bit parallel data in a byte serial decoded input/output record format. The control unit performs a data management function to maintain records of the location of all the data stored in the DASD unit. The DASD unit includes rotating data storage media consisting of a plurality of disks, each of which has associated therewith one or more moveable read/write heads. The data is stored on the rotating media in a track format that consists of a plurality of concentric rings of data. A control module is also included at the head of a string of disk drive units and connected to the control unit to convert between the eight bit parallel data format of the data channel and the bit serial data format of the rotating media. In addition, the control module controls data formatting and error correction code generation. The control module is connected by way of a bus to a plurality of read/write heads attached to an actuator, each of which serves to read and write data on an associated disk of the rotating media.
In a data record read operation, the processor transmits a seek request, through the channel interface unit and an available data channel, to the DASD control unit associated with the rotating media on which the requested data record is stored. The processor, upon completion of the transmission of the seek request, returns to processing other tasks. The DASD control unit responds to the seek request by determining the physical location of the requested data record on the plurality of disks. The control unit transmits information to the associated control module identifying the physical location and size of the data record in order to retrieve the requested data record from one of the disks of the rotating media. Since the data record is stored in a track format on a rotating media, the actuator associated with the identified disk of the rotating media on which the requested data record is stored must wait until the rotating media rotates a sufficient distance to present the beginning of the requested data record underneath the read/write head associated with the actuator.
A predetermined time before the beginning of the requested data record reaches the read/write head associated with the actuator, the control module requests the control unit to obtain a data communication path to the processor in order to transmit the retrieved data record from the rotating media directly to the processor over this data communication path. If a data communication path to the processor is not available, the control module must wait for one entire revolution of the rotating media before the media is again in the position where the requested data record is a sufficient distance away from the read/write head to establish a data communication path to the processor. This process is repeated until a data communication path to the processor is available and the data, as read by the read/write head, can be transmitted directly via the control module and control unit and an available data channel to the processor. It is obvious that there can be numerous delays in this data storage system while data communication paths are established. In a transaction based system, where there are a multitude of random data seeks, such delays can unnecessarily tie up actuators in the disk drive unit and significantly increase the system response time.